1. Field of the Invention
The present invention relates to an optical disk apparatus, and particularly to a technique for performing a gain adjustment using an equalizer on an RF signal read by an optical pickup.
2. Description of the Related Art
Related Art
Optical disk apparatuses for driving an optical disk, such as a CD (compact disk), a CD-R (compact disk-recordable), a CD-RW (compact disk-rewritable) and a DVD (digital versatile disk), are known. An optical disk apparatus reproduces data recorded on a disk by irradiating laser light onto the disk from an LD (laser diode) and converting the light reflected from the optical disk surface into an electric signal (RF signal). When the recording density per unit area is increased in such systems, such optical disk apparatuses suffer from a problem that signal amplitudes are reduced, especially in a high frequency region (near 3T), and the error rate therefore increases.
Thus, the RF signal supplied from an optical pickup is processed to remove noise by cutting signal components beyond a prescribed range of frequency bands using a low-pass filter, and in turn the resultant signal is processed so as to increase (boost) a gain in a high frequency region near 3T using an equalizer.
FIG. 4 shows frequency characteristics of one such equalizer. Referring to FIG. 4, the frequency is plotted on the horizontal axis and the gain is plotted on the vertical axis. In the equalizer, a gain is set to a certain value for increasing amplitudes near a 3T frequency f3T.
The RF signal with the gain adjusted by the equalizer is digitized by a digitizing circuit and provided therefrom as a reproduced signal. The digitized signal is subjected to processing for detection of a jitter component, and the result is sent to a control unit such as a CPU. The control unit functions to perform offset adjustment of a focusing servo and a tracking servo so as to minimize the jitter amount (phase difference).
The jitter amount is detected by generating a clock signal synchronized with the signal from the digitizing circuit using a PLL (phase-lock loop) circuit, and integrating or summing the phase difference between the components (3T to 11T) contained in the digitized signal and the clock signal.
As explained above, it is possible to increase a signal amplitude by boosting the RF signal near the 3T frequency by use of the equalizer and to reduce the error rate during reproducing. However, such boosting contributes to a time delay near the 3T, thereby an additional phase difference corresponding to the delay is caused and added to the original jitter amount. Thus the problem remains that the amount of jitter cannot be precisely detected, even if the phase difference between each component in the range of 3T to 11T of the digitized signal and the clock signal is integrated, and optimal adjustment of an offset amount of the focusing and/or tracking servo therefore remains impossible.
FIG. 5 shows the relationship between the offset amount and jitter amount of a focusing servo (FS) in cases when an RF signal is boosted and when not boosted. Referring to FIG. 5, the FS offset amount is plotted on the horizontal axis and the jitter amount is plotted on the vertical axis. A solid line represents the characteristic when the RF signal is boosted by the equalizer, while a dotted line represents the characteristic when not boosted. The integrated value of the phase differences of the all T components (in the range of 3T to 11T) presents a characteristic curve like a quadratic function which is symmetry with respect to the FS offset amount, and thus the FS offset amount that minimizes the jitter amount can be easily determined by calculating the intermediate value between the two points having a generally equal jitter amount to each other. In contrast, when the RF signal is boosted near the 3T component, the symmetry is lost, and, even if the intermediate value between the two points having a generally equal jitter amount is calculated, the FS offset amount which minimizes the jitter amount is not necessarily obtained and a deviation δ (about 0.3 μm) may occur. Accordingly, the FS offset amount cannot be set to its optimum value, and there arises a problem that the laser power must still be increased in order to correct the deviation of the focusing servo.